Apple and Google have defined strict design requirements to help all Passes look stunning and stay consistent.
Indeed even though you may be designing the pass using the PassKit web designer this is not 'exactly' how it will appear in Apple Wallet and Google Pay. PassKit translates your design online into code. Code that is recognised by Apple and Google. And then Apple and Google will display this in their wallet app appropriately.
Anything you cannot do in the PassKit Designer
is not possible within the Apple and Google framework.
PassKit ensures that you comply with those design standards so your Pass can be accepted into Apple Wallet, Google Pay and any other future wallets that may become mainstream.